This invention pertains to the use of laser heterodyne detection in interferometry, particularly as it relates to methods and apparatus for very accurately characterizing optical filters, measuring optical density of photographic plates or films, providing a well calibrated source of very low power laser radiation, and making measurements of the bidirectional reflectance distribution function (BRDF) and bidirectional transmittance distribution function (BTDF) of surfaces.
At the present time, instruments used for these purposes utilize direct, or incoherent detection, and incorporate detectors such as photomultipliers, photoconductors, or photodiodes. Problems inherent in such systems are of several kinds. In particular, they have a restricted dynamic range, e.g. the linear dynamic range of a photomultiplier is typically no more than five to six orders of magnitude. Photodiodes, the best of these detectors in terms of dynamic range, are linear over a range of about seven or eight orders of magnitude, but are limited in terms of sensitivity. Also, measurements are often desired over a much wider dynamic range than that which is available even with the photodiode. In addition, the photomultiplier, which has the greatest sensitivity, has the smaller dynamic range. Hence, accurate measurements can only be made over a limited dynamic range, and in practice the photomultiplier is used primarily as a null detector. (See W. Budde, Optical Radiation Measurements. Vol. 4, Academic Press, 1983, for a general reference regarding optical detectors.)
Another problem also often encountered using such incoherent detection schemes concerns angular resolution. In practice, it is difficult to achieve high angular resolution without encountering unacceptable losses in sensitivity. Also, such systems are not inherently polarization sensitive, and hence require additional elements to characterize the polarization of the detected beam.
What is needed is a detection system which has a significantly improved linear dynamic range and is also shot noise limited in sensitivity.